Facsimile (fax) machines have been growing in popularity in recent years; so much so that the growth has been labeled "spectacular" and "unprecedented." Much to the dismay of many courier services, fax machines have been gradually replacing overnight mail delivery for business communications, especially because the communication is instantaneous rather than delayed by several hours. Furthermore, as such business products migrate into the home office environment, and ultimately the home, personal communications will constitute a significant percentage of facsimile machine usage. For such migration to occur, however, the quality of document reproduction must increase while the cost of the machine must decrease.
Group III facsimile machines subdivide each line scanned into a number of black or white picture elements (pixels). It is important that the scanning equipment be able to accurately determine whether each pixel is predominantly white or predominantly black so that faithful reproduction of a document may be accomplished. Line scanning equipment generally includes multiple light sources for illuminating the line being scanned, as well as multiple lenses and sensors for use in measuring the light that is reflected from the individual pixels. Due to the normal variation found among individual, but presumed identical components, and due to manufacturing variations there will be nonuniformities in the reproduction of each document. Fortunately, such nonuniformities do not change significantly with time and may therefore be corrected at the time of manufacture and periodically thereafter. A number of facsimile scanners use charge coupled device (CCD) arrays having all light sensors on the same semiconductor substrate. This technique eliminates some of the variation in response among the light sensing elements, but is still subject to the variation in illumination across a line which may require correction.
In some facsimile scanners using photodiode arrays, an all-white document is presented to the line scanner for the purpose of calibration. The output voltage levels produced by the individual photodiodes are converted into digital signals, processed to form correction signals, and used during normal operation of the scanner to multiply the digital value of each new output voltage produced by the photo-diodes. This process, however, requires analog-to-digital conversion of each of the photodiode output voltages and multiplication of two digital numbers which is both costly and time consuming. Typically a large number of conversions and multiplications must be performed in very short order so that customized high speed components are required. The speed with which digital multiplication may be accomplished is improved by using a look-up table (Read Only Memory - ROM) wherein the product of multiplication is stored at a location jointly addressed by the multiplicand and multiplier factors. Indeed, rather than storing merely the product of two numbers within the ROM, the functions of storing correction coefficients and multiplication can be combined in a single ROM. Here, the ROM is jointly addressed by the number of the particular pixel being scanned anbd the A/D converted output of the line scanner for that particular pixel. The output of the ROM look-up table is the corrected, A/D converted output of the line scanner. Nevertheless, high speed analog-to-digital conversion of the line scanner's output analog signal is still required and somewhat costly to implement, especially for Group III facsimile transmission. Accordingly, a high-speed, low-cost circuit for correcting nonuniformities in pixel output level for a line scanner is desirable.